compare list

Brian van den Broek broek at cc.umanitoba.ca
Tue Nov 15 10:11:05 CET 2005


Ben Bush said unto the world upon 2005-11-15 01:24:

<top posting corrected and post trimmed>

> 
>  Unfortunately, the indents got screwed up along the way. But the part
> 
>>of my code you asked about was:
>>
>>for item in list1:
>>    if item in list2:
>>        if item + 1 in list1 and item + 1 in list2:
>>            return True
>>
>>In some detail:

<snip my detailed explanation>

>>Does that clarify it?
>>
>>Finally, your response to Alex would have been much more useful if
>>you'd quoted the error rather than just asserting that you got an
>>error :-)
>>
>>Best,
>>
>>Brian vdB

Hi Ben,

first, while there are those on the list/n.g. who differ, the 
majoritarian view is that top posting isn't a good thing. At minimum, 
if someone bothered to correct it, it would be nice if in a further 
follow-up you didn't top-post again :-)

Second, you might find the tutor list really helpful. It is where I 
learned most of what I know, and I still read it more than c.l.p. It 
is very newbie friendly.

As for your question:

 >> Hi Brian,
 >
 > regarding "if item + 1 in list1 and item + 1 in list2:",
 > my understanding is this will check whether the following item in 
each list
 > is the same. How does the code permit the situation that the order 
does not
 > matter?
 > For example, for lisA and lisB, the comparison is true and the two 
lists
 > have 5 and 6 but different order.
 >  lisA=[1,2,3,4,5,6,9]
 > lisB=[1,6,5]
 >  Many Thanks!


There are two distinct issues that might be the source of your 
confusion. I will be explicit about both; pardon if only one applied.

 >>> num_list = [1, 42, 451]
 >>> for item in num_list: print item, type(item)

1 <type 'int'>
42 <type 'int'>
451 <type 'int'>
 >>>

Iterating over a list as I did makes item refer to each list member in 
turn. So, item + 1 doesn't refer to the next item (except by accident 
as it were when two items are sequential ints). Rather, it refers to 
int that results from adding 1 to the int that is item.

You might be thinking of list index notation instead:

 >>> index = 1
 >>> num_list[index], num_list[index + 1]
(42, 451)
 >>>

(General tip: putting a print or two in my original code would have 
cleared that up very quickly.)

So, that cleared up, are you wondering why item + 1 suffices, instead 
of both item + 1 and item - 1?

If so, consider that it the list1 has both n and n - 1 in it and we 
iterate over list1 checking each case, eventually item will refer to n 
- 1. In that case, item + 1 = n and we are covered after all. I did as 
I did in my original code thinking it was probably quicker to have 
only one test and pay the cost that there might be a quicker exit from 
the iteration were I to test both item + 1 and item - 1. f it 
mattered, I'd test for speed. Of course, if speed mattered and I could 
ever remember to use sets :-) I'd follow Alex and Duncan's suggestions 
instead!

If that doesn't clear it up, give yourself a few short lists and run 
test cases having inserted print statements to see what is going on. 
To stop it all from whizzing by too fast, put in

raw_input("Hit enter and I'll keep working")

somewhere in the loops to slow things down.

HTH,

Brian vdB




More information about the Python-list mailing list